RF tags have come into use lately as a means of identification which can be remotely sensed. U.S. Pat. No. 6,563,417 describes an RFID tag mountable to a product or product carrier. The RFID tag contains different chips for computing, storage and sensing, as well as a battery and radio transceiver. The '417 patent also teaches a quite complex communication scheme; undoubtedly a very expensive tag.
There exists a need for a very inexpensive, even disposable, RF tag which can monitor one or more conditions in the environment and communicate in some manner when those conditions have exceeded some predetermined limit.